1. Field of the Invention
The present invention relates generally to backlight systems for illumination of large area displays. More particularly the invention concerns backlight systems for high dynamic range displays based on liquid crystal and micro-electromechanical light modulators.
2. Discussion of the Prior Art
Backlight assemblies provide a planar light source for transmissive displays. More common light sources used in display backlighting include cold cathode fluorescent lamps (CCFL) and light emitting diodes (LED's ).
Depending on the positioning of light sources, the backlight systems may be grouped either “edge-lit” or “direct-lit”. Edge-lit backlight systems typically include a light source, a thin rectangle shaped optical waveguide, a reflector, or a diffuser at the back of optical waveguide and prism films mounted between the waveguide and the display panel. The primary drawback of edge-lit backlight systems is their inability to illuminate large displays. Inefficient light coupling and concentrated heat generated from the light sources tend to limit the size of edge-lit backlight systems.
In direct-lit backlight systems, a plurality of light sources are generally evenly distributed along the display area and each of the light sources directly illuminates a small segment of the display. In order to achieve relatively uniform illumination, the light sources are typically assembled on a reflective panel and placed at a relatively great distance from the display. Additionally, a volume diffuser is often placed between the light sources and the display panel.
High dynamic range displays typically include two displays, the first display functioning to modulate the light and to illuminate the second display that further modulates light. For example, in high dynamic range LCD displays having LED backlights, the LEDs are modulated to provide an area-adaptive backlight. Currently, high dynamic range LCD displays use direct-lit backlight systems of the character previously described. These systems typically exhibit several problems, including the fact that each LED illuminates the display segment of other LEDs. This is referred to as crosstalk. In this regard, each LED has its own crosstalk pattern which requires extensive computation and causes visual artifacts. Further, in such systems, display depth is increased as a result of the required distance from the LEDs to the display panel. Additionally, such systems tend not to be efficient due to the required multiple reflections from the reflector panel and the use of a volume diffuser.